Scenario Maintaining Shuttle Atlantis in orbit after launch

[richfororbit]

Hi,

I have decided to stay away from orbiting or short distance docking and have moved back to the shuttle Atlantis scenario. Once I have reached orbit and the boosters have detached from the hydrogen tank, what is the technique in creating an orbit. This isn't the XR2, I only have used that once for reaching Martian orbit from the surface.:thumbup:

Must I rotate the craft so the Earth is above the vessel? I got that from watching an old shuttle film dated back forty years...probably the wrong source for orbital technique paths.

I essentially tried to balance my altitude by keeping the nose leveled and then gliding.. but that didn't quite work, but it was progress as I maintained orbit much longer, as in previous attempts when I tried Orbiter out I failed very quickly into flight.

Thanks.

 

[Urwumpe]

Small hint: Stay in Orbit = keep perigee altitude high above kalman line (which is in 100 km altitude). 250 km PeA will do that well enough for government work.

The shuttle is usually reaching orbit in a defined suborbital trajectory, so that the external tank does fall back to Earth and the Shuttle can safely land again, should there be any problem with the OMS. Which means that you always have to fire your OMS engines for reaching raising your perigee and stay in Orbit.

Also, the normal flight orientation of the Shuttle is: Engines face in direction of flight and payload bay points to Earth.

 

[jangofett287]

Also, the normal flight orientation of the Shuttle is: Engines face in direction of flight and payload bay points to Earth.

Sorry, is that exhaust vector faces along the velocity vector or the thrust vector along the velocity vector?

 

[richfororbit]

So I must use the thrusters to help create the circular orbit? When I wasn't careful with the orbit, the altitude went to 350. Government work?

 

[PhantomCruiser]

You have a high point of your orbit and a low point of your orbit, the difference results in eccentricity. Have both points above 200Km (at least) but like Urwumpe mentioned 250 is better.

After ET sep, coast until about 30 seconds before PeA, and (oriented prograde) perform an OMS burn to circularize.

Oops, That Apa, NOT PeA...

 

[Urwumpe]

Sorry, is that exhaust vector faces along the velocity vector or the thrust vector along the velocity vector?

It means, that the business end of the Space Shuttle points forward in orbital flight.

 

[orbitingpluto]

I'd like to point out that Phantom Cruiser made a small mistake, you ought to make your burn at your ApA. You can't raise your perigee unless your at your apogee, so burning at PeA would mean you knee-deep in the atmosphere raising the wrong part of the orbit.

They're both similar three letter acronyms, and since I've had a bit minor trouble typing one while intending the other just now, I can see how it might have happened.

 

[richfororbit]

I actually did I think a little better, but I left the fuel tank attached...as well my atlitude dropped below 100KM As for PEA and all that well the numbers were jumping around. I made sure that the cargo bay doors are facing the Earth.

 

[DaveS]

I actually did I think a little better, but I left the fuel tank attached...as well my atlitude dropped below 100KM As for PEA and all that well the numbers were jumping around. I made sure that the cargo bay doors are facing the Earth.

For a successful OMS2 burn, the the nose needs to face forward so that the thrust direction is prograde. Roll doesn't matter.

You really need to get brushed up on your orbital mechanics, so I suggest that you go here: http://www.orbiter-forum.com/tutorials.php

 

[richfororbit]

I tried that all last year, and I gave up. So I learnt only easy steps, I guess, well nothing is really easy in Orbiter.

Did have the fuel tank empty actually help destroy my orbit when I was doing the prograde burn, even prior I just tried an engine burn which made a little different to my orbit.

 

[boogabooga]

Yep. What DaveS said.

Orientation does not matter to your orbit. That should have been made clear. The cargo bay facing Earth thing is just incidental.

The entire purpose of an orbit is that it doesn't need to be maintained except over longer time scales (weeks, months, years).

It's not that you can't maintain an orbit, your problem is that you are not entering into a proper orbit to begin with.

Also try this:
[ame="http://www.youtube.com/watch?v=L-zaubu3RgU&index=2&list=PLPu_SHPaJzs4e1NfhY8jxPGgk5S4duEql"]Orbiter 2010 - [Part 1] Absolute Beginner Guide - Getting to Orbit 1 - YouTube[/ame]

 

[DaveS]

I tried that all last year, and I gave up. So I learnt only easy steps, I guess, well nothing is really easy in Orbiter.

Did have the fuel tank empty actually help destroy my orbit when I was doing the prograde burn, even prior I just tried an engine burn which made a little different to my orbit.

You need to jettison the ET before the the OMS2 burn. Normally ET sep occurs about 20 seconds after MECO.

 

[richfororbit]

MECO? What!! :embarrassed:

But that is the XR2. I would prefer an actual Shuttle clip, but there are hardly any.

 

[DaveS]

MECO? What!! :embarrassed:

But that is the XR2. I would prefer an actual Shuttle clip, but there are hardly any.

You really should read through Go Play In Space as your knowledge about orbital mechanics is near or at zero.

You're not ready for the shuttle.

 

[Krikkit]

What inclination are you using? You will waste a lot of fuel trying to orbit at an inclination below your launchpad. Remember orbiting is mostly about velocity, not altitude. I have had Atlantis maintain a orbit at altitudes below 150km. You need to plan your trajectory so that you reach orbital velocity before MECO (Main Engine Cut Off).

 

[richfororbit]

As Mr Cernan said many years ago wanting to be an Astronaut you have to be arrogant. Well I'm not going anywhere, but as far as the Simulator I'll keep trying with or without a good understanding of those mechanics. Maths and Orbital Mechanics isn't easy for reading for me.

I'm notsure about anyone else here.

 

[Donamy]

Some near shuttle addons are available with auto-pilots. You could observe those.

 

[ADSWNJ]

MECO? What!! :embarrassed:

But that is the XR2. I would prefer an actual Shuttle clip, but there are hardly any.

Hi Rich - let's start with something a bit easier, and then work back to the Shuttle. Jump in a Delta Glider, and default Cape Canaveral scenario, and get it up to orbit. Don't worry about flying through buildings, or no runway ... just full throttle, get airborne, raise gear and you are on your way. The benefit of the Delta Glider is that it has unrealistically (or 'future') powerful engines, so you can concentrate on the Orbit MFD and not worry too much about everything else.

You want to take off, and pitch to say 10-20 degrees nose-up and keep that attitude into orbit. You want to watch three numbers on the Orbit MFD: Alt, ApA and PeA. (If you have ApR and PeR instead, then hit the DST button to switch from radius measurements to altitude measurements.)

Your goal is to get to roughly 7750 m/s horizontal speed, at say 250km altitude. The 250km altitude is nicely outside the practical atmosphere (bar the odd molecule or two), and the 7500 m/s (give or take) is enough speed to get your centrifugal force to counteract gravity.

Back to those 3 numbers ... you'll notice that as your altitude (Alt) increases, the apoapsis (ApA) stays very close to it as you speed up. 3000 m/s ... 4000 m/s ... 5000 m/s etch ... it'll be close. This means that if you cut the engines at this point (main engine cut off = MECO), then the highest altitude you will get is roughly your current altitude.

As you head through 7000 m/s, watch how the ApA starts to run away from the Alt. This is your indication that you are nearing orbital velocity, and if you are not careful, you will escape Earth's gravity completely. Try throttling back to say 10-20% thrust to keep the ApA ticking over at a slower rate.

When the AoA hits your desired level (e.g. 250km), then shut off all thrust. Depending on your actual altitude (e.g. < 120km or higher), you may see the ApA degrade a bit due to friction with the atmosphere, but most likely you will coast up to close to your desired ApA.

Have a look at your PeA at this point. If your ascent was relatively steep (i.e. pitched up aggressively), then your PeA could be significantly negative (e.g. -1 mega-meter or lower). Or it could be anything up to close to your AoA. It's all how you finesse the angle of the ascent. But let's say it's negative. This means that you are currently sub-orbital, like an inter-continental ballistic missile, and once you go through the top of your arc, your trajectory will bring you back to Earth in a very rapid fashion.

This is no problem at all. You just need to select the Prograde autopilot) say 200 seconds before you get to the ApA point. (How do you know when this is? Simple ... just watch the time to ApA tick down on the ApT timer). So you are in Prograde aligned to the velocity vector, and you are ticking down to the ApA point. At say 15 seconds to go ... hit the throttle again, and watch the PeA raise up. Keep going until it's at least 120km, and ideally all the way to what you want (e.g. 250km).

As your orbit becomes more circular, the eccentricity (Ecc value) will drop to close to 0.0000. Don't worry if it's say 0.0030 or less ... that's good enough for now.

The principles are the same in the Shuttle, but you have much less thrust authority, so you have to plan ahead more, be more precise on your trajectory, and thrust for longer.

Hopefully this gives you some quick pointers. I hate to see anyone get frustrated with these core skills, as once you get the knack, you will be much more confident to make the simulation do what you want it to do.

 

[PhantomCruiser]

I'd like to point out that Phantom Cruiser made a small mistake, you ought to make your burn at your ApA. You can't raise your perigee unless your at your apogee, so burning at PeA would mean you knee-deep in the atmosphere raising the wrong part of the orbit.

They're both similar three letter acronyms, and since I've had a bit minor trouble typing one while intending the other just now, I can see how it might have happened.

Yep, sometimes I think faster than I can type.

It also goes to prove what I've said on one of the other threads regarding terminology. Knowing the lingo means you can ask better questions, resulting in better answers.

 

[Matias Saibene]

I recommend running the tutorial scenario "Space Shuttle to ISS" that will help you learn some maneuvers:thumbup:.